Not applicable.
In many cryogenic fluid transfer applications, it is important that the fluid be transferred in a 100% liquid state, or as close to 100% as possible. Conventionally, this required the fluid to be initially phase-separated and/or subcooled in a heat exchanger and/or vacuum jacketing the line to keep it well insulated. Otherwise, the heat leak in the transfer line would cause boil-off, thereby causing flow undulations in the transfer line and resulting in a non-steady, pulsing and generally undesirable flow. Heat leak is particularly a problem for long transfer lines.
The present invention addresses this first concern for cryogenic transfer lines with a coaxial or xe2x80x9ctube-in-tubexe2x80x9d geometry where a first portion of the cryogenic fluid flows through the inner tube while a second portion flows through an annulus between the inner tube and outer tube which annulus is at a lower pressure than the inside tube. By virtue of this pressure differential, one skilled in the art can appreciate that the liquid in the annulus can provide a refrigeration duty to the liquid inside the inner tube (e.g. such as by boiling) such that this inner liquid is cooled and stays a saturated liquid. Preferably, the liquid is even subcooled slightly such that a xe2x80x9ccushionxe2x80x9d of refrigeration is available to fight heat leak.
It is also important in many cryogenic fluid transfer applications that the transfer line be lightweight and flexible. This provides for maximum degrees of freedom during installation, operation and maintenance and also enables the line to withstand repeated bending. The present invention addresses this second concern for cryogenic transfer lines by making at least a portion of the line out of a flexible polymeric material.
The prior art does not provide for a cryogenic fluid transfer line that addresses both of these important concerns.
U.S. Pat. No. 3,696,627 (Longsworth) teaches a liquid cryogen transfer system having a rigid coaxial piping arrangement for subcooling and stabilizing cryogen flow during transfer. U.S. Pat. No. 4,296,610 (Davis), U.S. Pat. No. 4,336,689 (Davis), U.S. Pat. No. 4,715,187 (Stearns) and U.S. Pat. No. 5,477,691 (White) teach similar systems.
Chang et al. teaches non-metallic, flexible cryogenic transfer lines for use in cryosurgical systems where the cryogen is used to cool the cryoprobe in a cryosurgical system (xe2x80x9cDevelopment of a High-Performance Multiprobe Cryosurgical Devicexe2x80x9d, Biomedical Instrumentation and Technology, September/October 1994, pp. 383-390). Due to the heat leak boil-off resulting from the design of the flexible lines in Chang, combined with intrinsically poor insulation, such lines must be short and fed with a substantially subcooled cryogenic liquid (e.g. liquid nitrogen at xe2x88x92214xc2x0 C.) in order to work properly. This requires the up-stream usage of complex and expensive cryogenic storage, supply and control systems.
Cryogenic transfer lines are also taught for use in machining applications where the cryogen is used to cool the interface of the cutting tool and the workpiece. See for example U.S. Pat. No. 2,635,399 (West), U.S. Pat. No. 5,103,701 (Lundin), U.S. Pat. No. 5,509,335 (Emerson), U.S. Pat. No. 5,592,863 (Jaskowiak), U.S. Pat. No. 5,761,974 (Wagner) and U.S. Pat. No. 5,901,623 (Hong). Similar to Chang, such lines must be short and fed with a substantially subcooled cryogenic liquid to combat heat leak boil-off and thus requires an expensive up-stream subcooling system.
U.S. Pat. No. 3,433,028 (Klee) discloses a coaxial system for conveying cryogenic fluids over substantial distances in pure single phase. Using fixed-size, inlet orifices in the cryogenic-conveying inner line, the liquid is admitted to the outer line where it vaporizes when subject to an external heat leak. A thermal sensor-based flow control unit, mounted at the exit end of this coaxial line, chokes the flow of the vapor in the outer line depending on the value of temperature required, usually 50 to 100 deg. F. more than the boiling point of the liquid in the inner line. As a result, the outer line pressure may be near the cryogenic source pressure, and its vapor always will be warmer than the inner line liquid. Moreover, high heat leaks cannot be fully countered since the amount of liquid admitted to the outer line for evaporation is permanently limited by the fixed-size inlet orifices. These operating principles necessitate the use of high-pressure resistant, non-flexing metal tubes and a thick-wall thermal insulation in the construction of the line.
JP 06210105 A teaches a polymeric coaxial transfer line for non-cryogenic degassing applications. The tube material characteristics preclude the use of the transfer line in cryogenic applications.
The present invention is a method and apparatus for transferring a cryogenic fluid. A polymeric, coaxial (i.e. xe2x80x9ctube-in-tubexe2x80x9d geometry) transfer line is utilized where a first portion of the cryogenic fluid flows through the inner tube while a second portion flows through an annulus between the inner tube and outer tube which annulus is at a lower pressure than the inside tube. In one embodiment, the inner tube is substantially non-porous and the transfer line is preceded by a flow control means to distribute at least part of the first and second portions of the cryogenic fluid to the inner tube and annulus respectively. In a second embodiment, a least a portion of the inner tube is porous with respect to both gas permeation and liquid permeation such that both a gaseous part and a liquid part of the first portion permeates into the annulus to form at least a part of the second portion.